Restoration is the re-establishment of trunk-bearing carrier groups after loss of all or most of the physical transmission facility between two sites, through geographical rerouting via redundant network capacity. Rerouting for restoration of the physical transport network should not be confused with the routing of individual calls or with the architecture of the logical trunking network administered at the DS-1 transport level. These will remain unchanged by a successful restoration at the DS-3 level.
As a cable-based technology, FOTS has proven susceptible to frequent damage due to construction work, lightning strikes, rodent damage, craftsperson error, train derailment, etc. With the inherent capacity of FOTS, and reduced physical route diversity in fiber-based networks, a cable cut can seriously affect network blocking because each fiber carries many diverse logical trunk groups. Structural availability of fiber cable in real networks is reported as low as 96.5% (300 hours/year downtime) at a cost up to $75,000 US per minute of outage. By comparison, radio transcontinental routes typically do meet requirements of 99.985% for structural availabilities without special restoration methods. It therefore seems unavoidable that advanced methods of restoration are an essential adjunct to the widespread deployment of fiber networks.
The conventional method for restoration of a fiber cable cut is manual rearrangement at passive DSX-3 crossconnect panels. The sequence of patch operations is determined from stored plans or is developed at the time of restoration. Crews are dispatched first to patch the rearrangements and then to proceed with physical fault location and repair. The time to restore traffic averages from 6 to 12 hours.
The current industry plans are that with the advent of DCS 3/3 (or DCS 3/1) machines, the above restoration method will be automated through remote control of DCS-3 machines. Some of the problems and limitations with the planned approach are:
Speed: Although centralized control of DCS-3 machines will significantly reduce restoration times compared to manual patching, there is little expectation of approaching realtime reconfiguration capabilities with centralized control. Estimates within the industry place restoration times initially around 1 hour, improving to perhaps 10 minutes when the centralized DCS management systems are mature.
Database Dependency: The centralized approach raises concern about the size, cost, complexity and vulnerability of the surveillance and control complex that will be needed for transport management. The centralized approach will be dependent on the ability to maintain a complete, consistent, and accurate database image of the network over years of operation. Eventually a maintenance change that is not immediately and correctly reflected in the database creates the possibility of service-affecting error during a centrally controlled restoration or reconfiguration event.
Traffic Impact: With centralized control, all calls-in-progress will continue to be lost whenever a cable is cut because the outage duration remains much longer than voice call-dropping thresholds and data protocol timeouts. This means that with centralized control, span protection switching will continue to be required in transmission systems to handle single carrier failures with enough speed to avoid call dropping.
Telemetry Dependency: Centralized control of DCS machines also requires redundant telemetry arrangements so facility cuts will not remove the very communications links over which the central control site is to issue restoration commands. This requires redundant communications interfaces on the DCS equipment and special circuit engineering considerations for the operating company.